I am in a bit of a hurry, because I wanted to test the bed out on a trip to South Florida during the holidays. There is a lot of work, but fortunately much of the bed frame was pre-assembled of the last few weeks in my workshop.
The width and length of the bed were mostly dictated by the size of the van. I choose 32″x 70″; small enough for the van, large enough to have a good night’s sleep. Height was based on the content of the compartments underneath. Specifically the batteries and the toilet. I bought a toilet that was a little under 13″, but I planned a sliding panel on top of that. I decided that 15½” height should be sufficient. I figured out that the top of the bed would stay just underneath the window; the mattress on top of that would cover it partially, but that was not a major problem. The original plan asked for a new cargo van, in which case I would have installed a smaller window.